Background: The Repeat Expansion Diseases are caused by intergenerational expansions of a specific tandem repeat. More than 20 such diseases have been identified thus far. Expansion of a CGGCCG-repeat in the 5' UTR of the FMR1 gene is associated with 3 different clinical presentations: Individuals with 60-200 repeats, the so-called premutation allele, are at risk for Fragile X-associated tremor-ataxia syndrome whose symptoms include in addition to neurodegeneration, loss of autonomic function including bowel and urinary incontinence. Female carriers of premutation alleles are also at risk of premature ovarian failure. Furthermore, in females, the premutation allele can undergo expansion on intergenerational transfer that can result in their children having alleles with >200 repeats. This expanded allele is known as a full mutation and individuals who inherit such alleles almost always have Fragile X mental retardation syndrome (FXS), which is associated with autistic symptoms, digestive difficulties, and behavior problems including aggression and depression. The mechanism by which is expansion occurs is unknown. It is thought to differ from the generalized microsatellite instability seen in many different cancers in that the instability is confined to a single genetic locus, shows a strong expansion bias and actually requires genes the mismatch repair pathway. Repeat expansion diseases caused by small increases in repeat number frequently show a paternal bias, while those diseases resulting from large increases in repeat number are frequently maternally derived. Whether this reflects 2 different mechanisms is unknown. The pattern of expansions are very different from what is seen in bacteria, yeast and mammalian somatic cells. Thus an animal model is much more likely to be useful in understanding the events responsible for the repeat expansion mutation.[unreadable] [unreadable] Progress report: We have generated FXS premutation mice containing 120 CGGCCG-repeats in the 5 UTR of the endogenous murine Fmr1 gene (Entezam et al., 2007). We have shown that the repeat in these animals shows instability resembling the instability seen in human carriers of similarly sized alleles in its expansion bias and high frequency. However the average increase in repeat size with each generation is much smaller in mice than in humans. In humans, almost 100% of the progeny of females with >100 repeats have alleles in the full mutation range. We show for the first time that large expansions are possible in mice but they occur at a frequency of <1%. We have shown that the average size of each expansion increases as the parental repeat number increases suggesting that one reason for the lower frequency of large expansions in mice might be that mice have a higher threshold for such events than do humans. Data obtained by crossing these mice to mice with mutations in various DNA replication, repair and recombination genes suggests that 2 expansion mechanisms exist in mammals. One mechanism occurs predominantly on paternal transmission and involves small changes in repeat number. This mechanism, which may account for those human diseases caused by small expansions, probably occurs because of problems with DNA replication. The second expansion mechanism occurs during gametogenesis and is related to aberrant repair of DNA damage. Such expansions show a maternal bias in mice and may account for those repeat expansion diseases that involve large intergenerational increases in repeat number. It may also account for the differences in the frequency of large expansions in humans and mice: Since human oocytes spend decades in the post-replication phase of gametogenesis compared to either female mice or males of both species, the gametes of human females may be at much higher risk of incurring the DNA damage responsible for large expansions.